Astronomers Discover 'Super-Earth' Orbiting Nearby Star

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European astronomers report the detection of a new
extrasolar world several times more massive than Earth. The newly found
exoplanet, classified as a so-called "super-Earth," is circling a
nearby star designated GJ 625. The researchers detailed their finding in a
paper published May 18 on arXiv.org.

"Super-Earths" are planets more massive than Earth
but not exceeding the mass of Neptune. Although the term
"super-Earth" refers only to the mass of the planet, it is also used
by astronomers to describe planets bigger than Earth but smaller than the
so-called "mini-Neptunes" (with a radius between two to four
Earth-radii).

Located only 21 light years away, GJ 625 is an
M-dwarf star (spectral type M2) about 1/3 the size and mass of the sun. Such stars offer great potential in the
search for alien Earth-like worlds, as low-mass rocky planets appear to be more
frequent around M-dwarfs.

However, finding an Earth-like planet orbiting
low-mass stars is difficult due to their stellar activity. This is because
signals induced by the rotation of a star can easily mimic those of planetary
origin. Therefore, the signals coming from M-dwarfs tend to be comparable to
those of rocky planets close
to the habitable zone of their stars.

Recently, a team of astronomers led by Alejandro
Suarez Mascareño of the Canary Islands Institute of Astrophysics, has completed
challenging studies of GJ 625, which began in 2013 and lasted over three and a
half years. The observations were conducted with the High Accuracy Radial
velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph
installed at the Telescopio Nazionale Galileo at the Roque de los Muchachos
Observatory on the island of La Palma, Canary Islands, Spain.

Phase folded curve of the planetary signal detected in GJ 625 using the parameters of the MCMC model. Left panel shows the CCF measurements, right panel the TERRA measurements. Grey dots show the measurements after subtracting the detected activity induced signals. Red dots are the same points binned in phase with a bin size of 0.1. The error bar of a given bin is estimated using the weighted standard deviation of binned measurements divided by the square root of the number of measurements included in this bin. Blue line shows the best fit to the data using a Keplerian model. Credit: Mascareño et al., 2017.

The researchers analyzed 151 radial-velocity time
series from HARPS-N as part of the HArps-n red Dwarf Exoplanet Survey (HADES)
radial velocity program, which resulted in the discovery of a new planet.

"We report the discovery of a super-Earth
orbiting at the inner edge of the habitable zone of the star GJ 625 based on
the analysis of the radial-velocity (RV) time series from the HARPS-N
spectrograph, consisting in 151 HARPS-N measurements taken over 3.5 yr,"
the paper reads.

The newly found alien world, designated GJ 625 b,
has a minimum mass of 2.8 Earth masses, which makes it the lightest exoplanet
found around an M2 star to date. The planet orbits its host every 14.6 days at
a distance of approximately 0.08 AU from the star, which is relatively close.
By comparison, Mercury orbits the sun at a mean distance of 0.38 AU.

The results of radial velocity measurements allowed
the team to conclude that GJ 625 b is a small "super-Earth" on the
inner edge of the habitable
zone and has a mean surface temperature of 350 K that is very
dependent on the atmospheric parameters. Moreover, the researchers assume that
the newly detected exoworld might potentially host liquid water, but more
observations focused on GJ 625 b's atmosphere are required to confirm this
assumption.